The transmembrane portion of purified subunit c of the E. coli F1F0 ATP synthase in chloroform-methanol-water solution has been modeled based on assigned secondary structure, interhelical NOEs between assigned protons, and distances derived from relaxation effects of a covalently attached nitroxide. The protein has been incorporated into deuterated detergent micelles, and the conformation of the conserved "polar loop" is being determined in the detergent solubilized protein. The pKa's of the aspartic acid side chains have been measured, and the functionally crucial Asp61 has been found to titrate at much higher pH than the other carboxyl groups. The entire protein has been uniformly labeled with 13C/15N, and triple resonance data is being used to complete the assignments of the protein and get more complete long range NOE information.